1. Field of the Invention
The present invention is directed to duplexer/diplexers, and in particular to duplexer/diplexer filters.
2. Discussion of the Related Art
Duplexers (transmission/reception duplexers) are devices for the separation of transmission and reception channels in systems employing a common transmission and reception antenna, in order to prevent energy from flowing into the receiver during transmission or to prevent energy from flowing into the transmitter during reception. Such duplexers are presently used in analog mobile radiotelephone systems, and their use is planned for future digital systems having high data rates.
Whereas duplexers have two branches, namely a transmission branch and a reception branch, duplexers are suited for separating different bands of different mobile radiotelephone systems (for example, DCS1800/PCS), and thus potentially may have more than two branches.
Specifically in mobile radiotelephone systems, the following demands are made of a duplexer that usually comprises two ceramic filters:
Optimally, insertion attenuations of the two branches should be low, in the transmission branch for example, in order to lose as little transmission power as possible in the filter itself. Selection between the two branches, however, should be optimally high in order to suppress generation of mixed products in the two branches. In addition to high suppression of mixed products, which are extremely undesirable precisely given a mobile radiotelephone device, the filters themselves should be as small as possible so that the duplexer does not occupy too much space.
Currently, there are two fundamentally different types of duplexers comprising ceramic filters.
The first type, which are referred to as monolithic duplexers, comprise a ceramic member wherein a transmission branch and a reception branch include a plurality of coupled resonators. The advantage of such a monolithic duplexer involves its manufacture; specifically only one ceramic member need be pressed in one piece, which considerably simplifies the manufacture compared to the production of two ceramic members. One disadvantage that should not be underestimated, however, is that the ceramic members of such monolithic duplexers are hard to solder due to their size, whereby corresponding mechanical stresses often arise on the supports or xe2x80x9cboardsxe2x80x9d, since the ceramic filters themselves are not flexible. Given monolithic duplexers, the harmonic behavior tends to be poorer compared to duplexers made up of separate filters, since the square-wave waveguide modes that fundamentally always occur, already become capable of propagation at low frequencies, which has an especially disadvantageous effect for a monolithic duplexer.
In the other type of duplexer, what are referred to as ceramic filters or ceramic line resonators, are coupled or mounted on a carrier substrate. Coupling structures between the ceramic filters are either contained in the carrier substrate or are established by additionally provided coils and capacitors. A critical disadvantage of this type of duplexer involves high cost outlay for the coupling structures.
Further, a reduction of the structural height due to the additional carrier substrate occurs at the expense of deterioration of the electrical parameters such as, in particular, the insertion attenuation.
It is an object of the present invention to provide a duplexer/diplexer that is simple to manufacture and to mount, and that can be easily matched to desired applications.
This object is inventively achieved in a duplexer/diplexer wherein filters are modularly constructed and connected by shielding, and wherein the two branches are matched to an identical value of resistance in their passband and offer broadband high-impedance in the stopband of the band neighboring the passband, so that all three ports of the duplexer/diplexer are matched to the value of resistance given parallel connection of the two branches.
It is another object of the invention to provide a duplexer/diplexer, that is modularly constructed. In a duplexer, the two branches are thereby arranged such that they are respectively matched to a resistance of 50 Ohms in the passband and are rotated high-impedance or, offer broadband no-load in the stopband of the band neighboring the passband. In other words, all three ports of the duplexer are matched to a resistance of 50 Ohms given parallel connection of the two branches.
If one of the two branches is not already high-impedance in the stopband based on its very structure, it can easily be rotated into no-load by means of a line structure at the side of the antenna port corresponding to its length in the xe2x80x9cSmith diagramxe2x80x9d, i.e. the imaging of the right half of the impedance level onto the level of the complex reflection factor.
It is a further object of the invention to provide a duplexer/diplexer, wherein two or more branches can be fabricated independently of one another.
When the inventive duplexer is provided with roughly the same dimensions as an existing monolithic duplexer, as was explained above, then the two individual branches are each about half as large given the modular structure. By roughly double the frequency, the modular structure is significantly more advantageous with respect to the propagation capability of the first surface.
When, for example, there are limitations on the structural height, then there is the possibility of constructing the branches separately, thereby miniaturizing the reception branch, whereas the transmission branch is left as high as possible for maintaining the low insertion attenuation. Thus, a user is given latitude in the design.
All filters or duplexers/diplexers having coupled xcex/4 resonators have the disadvantage that they become capable of propagation given the 3-fold frequency or a corresponding mixed product thereof. This situation can be advantageously alleviated by a side structure located at a distance of xcex/12 from the front side of the respective filter or branch of the duplexer/diplexer.
Each filter is comprises a ceramic member wherein respective through bores are arranged that lead from one side of the respective ceramic member to the opposite side thereof, whereby terminal surfaces for three ports for the capacitative infeed/outfeed of RF signals insulated from a metallization of the ceramic member are provided on the ceramic member.